This application hereby claims priority of the French patent application FR 9915215 filed on Dec. 2, 1999.
The invention relates to the field of applying heat treatment to a fluid, and in particular heat treatments including at least one step of resistive heating.
Although a very wide range of fluids can be concerned by such treatment, the invention relates more particularly to fluids in the food industry, and in particular those that need to be pasteurized or sterilized, for example.
Resistive heating is a well-known technique for heating throughout a volume by the Joule effect. It consists in setting up an electric current in an electric circuit that terminates at electrically-conductive plates, and in causing an electrically-conductive fluid to flow between the plates. Since the fluid presents a certain amount of electrical resistance, it produces heat by the Joule effect, and consequently xe2x80x9cheats itselfxe2x80x9d.
Patent document FR 94/08108 discloses a resistive heater comprising a tubular central channel with planar electrodes placed at each of its two ends, the electrodes being pierced to enable a fluid to penetrate into the tube and to be collected therefrom. Those two electrodes are perpendicular both to the channel and to the general flow direction of the fluid.
An object of the invention is to provide a solution that differs from known solutions.
To this end, the invention provides a resistive heater comprising at least one heater chamber defined by walls of which two are constituted by substantially parallel conductive plates that are spaced from each other by a selected distance. The chamber also comprises at least one inlet enabling the fluid to be heated to be introduced close to a first end of the plates, and at least one outlet placed close to the second end of the plates, opposite from the first end, and enabling the fluid to be collected after it has flowed between the plates, substantially parallel thereto. Means are also provided for feeding electricity to the plates so that the fluid heats in the chamber by the resistive effect as it flows parallel to the plates.
As a result, firstly a large volume of fluid can be treated, secondly a large amount of heating can be obtained by acting on the dimensions and the spacing of the plates, thirdly the amount of electrode clogging is low, and fourthly the heater is easy to clean.
In a preferred embodiment, each chamber of the heater comprises at least one spacer which defines the space between the plates and comprises a hollow central portion allowing the fluid to flow together with two side faces against which the plates are placed, which side faces are provided with openings to allow surface contact between the fluid and the plates.
Under such circumstances, it is particularly advantageous for the spacer to comprise, at opposite ends of the central portion, respectively a first end portion in which the fluid admission inlet is formed communicating with the hollow central portion, and a second end in which the fluid collection outlet is formed communicating with said hollow central portion.
Depending on requirements, the heater may comprise a single chamber or a plurality of chambers juxtaposed one beside another in leakproof manner. In a xe2x80x9cseriesxe2x80x9d, first configuration, the chambers are juxtaposed in such a manner that the outlet of one chamber feeds the inlet of the following chamber, while the inlet of said chamber is itself fed by the outlet of the preceding chamber. The heater can thus be modular. In a xe2x80x9cparallel/seriesxe2x80x9d, second configuration, the chambers are juxtaposed beside one another in leakproof manner so that their respective inlets communicate with one another and their respective outlets communicate with one another. More preferably, the heater comprises a first set of chambers and at least one second set of chambers, the outlet from one of the first and second sets feeding the inlets of the other of the second and first sets. Any combination of these two configurations can be envisaged.
Each chamber may comprise one or two or even more juxtaposed spacers, in particular for the purpose of varying the spacing between the electrodes.
It is also possible to envisage chambers having two or more inlets, and one or two outlets, or more outlets, so as to enable two or more flows to flow simultaneously.
The invention also provides fluid treatment apparatus incorporating the above-described resistive heater. More precisely, the apparatus comprises a heater for heating a first fluid coupled to a first heat exchanger having a first circuit in which the heated first fluid from the heater flows, and a second circuit in which a second fluid flows, the first and second circuits being placed relative to each other in such a manner that the first and second fluids exchange heat so as to lower the temperature of the first fluid and increase the temperature of the second fluid by respective selected amounts.
In a first embodiment of the apparatus, which has only a single heat exchanger portion, the first fluid is the heated fluid delivered at the outlet of the heater, while the second fluid is a cooling fluid.
In a second embodiment of the apparatus, the outlet of the heater still feeds the inlet of the first circuit of the first heat exchanger, however the outlet of the second circuit of said heat exchanger feeds the inlet of said heater. The first fluid is thus the fluid that has been heated by the heater while the second fluid is the fluid to be heated by the heater. The first heat exchanger thus serves simultaneously to pre-heat the fluid and pre-cool the same fluid after it has been heated.
In this second embodiment, the first heat exchanger is preferably housed between the heater and a second heat exchanger. The second heat exchanger has a third circuit through which the pre-cooled first fluid delivered from the outlet of the first circuit flows, and a fourth circuit through which a cooling third fluid flows, the third and fourth circuits being placed relative to each other in such a manner that the first and third fluids exchange heat so as to lower the temperature of the pre-cooled first fluid by a selected amount.
Each heat exchanger is preferably of the stacked plate type. The successive pairs of plates define fluid flow chambers, and the successive chambers define portions of two different circuits so as to enable heat to be exchanged between the fluids of the two circuits.
The first and second heat exchangers could constitute a single overall heat exchanger. In which case, it is advantageous for the stack of plates in the general heat exchanger and the heater chambers of the heater to present dimensions that are substantially identical. This enables the general heat exchanger and the heater to be assembled together in series so as to form a one-piece structure using fastening means such as tie bars associated with nuts.
However, the heater and the heat exchanger(s) could be physically separate, with coupling between them being obtained by connecting pipes.
The invention also provides a method of treating fluid by resistive heating, which method comprises the following steps.
In a first step, one or more heater chambers is/are provided, each comprising two walls constituted by conductive plates that are substantially mutually parallel and spaced apart from each other by a selected distance.
In a second step, electricity is fed to the plates.
In a third step, the fluid to be heated is introduced close to a first end of the plates and the fluid is then caused to flow between the plates, substantially parallel thereto, so as to be heated inside the chamber by the resistive effect, and finally the heated fluid is collected from close to a second end of the plates, opposite from the first end.
In particularly advantageous manner, after the third step, a fourth step can be provided of lowering the temperature of the first fluid by a selected amount by exchanging heat with a second fluid.
In a first application, during the fourth step, the first fluid is the heated fluid delivered from the outlet(s) of the heater chamber(s), while the second fluid is a cooling fluid.
In a second application, during the fourth step, the first fluid is the heated fluid delivered by the heater chamber(s), while the second fluid is the fluid that is to be heated by said heater chamber(s). Thus, the fluid is simultaneously pre-heated and pre-cooled after it has been heated.
In this second application, the method may comprise, after the fourth step, a fifth step of lowering the temperature of the pre-cooled first fluid by a selected amount by exchanging heat with a cooling third fluid.